One reason the flies could be lost is because the line is in constant motion, therefore there is a whip like action when it turns to come forward on the forward cast...if the flies catch any piece of the rock by the point of the hook, no matter how smooth they are, that causes loss of motion, and the whip becomes more intensified, perhaps causing the line to snap....Just try to keep your wrist from breaking as mentioned before.
Wrist break causes the tip of the rod to drop lower than the shoulder and the trajectory of the flies becomes downward, with the potential for them to hit rocks etc behind.